Discover heartless mob in command (6)
I believe the answer is:
rumble
'discover' is the definition.
(to rumble someone is to discover their secret)
'heartless mob in command' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'in' means one lot of letters goes inside another.
'command' becomes 'rule' (ruling is a kind of commanding).
'mob' with its middle taken out is 'mb'.
'mb' inserted within 'rule' is 'RUMBLE'.
(Other definitions for rumble that I've seen before include "uncover secrecy perhaps" , "Harlem gangs might" , "Discover the truth about" , "Make a low noise" , "Such a strip" .)